Medical cannulas, also known as hollow needles, are used by inserting them into a vessel or into the skin to create access into the body of a living organism typically with hypodermic syringes in which case they are also called hypodermic needles, or with appropriate catheters. On the penetration side they are provided with a chamfer called the cut which has a specific cut-geometry. The customary cut-geometries are the facet cut for most clinical applications and the undercut which is preferably used for peripheral IV (intravenous) catheters.
The insertion of the cannula into the skin or into the vessels is predominantly felt to be painful by the patient and this has caused widespread anxiety and inherent aversion to the application of the syringe or the catheter. This pain and therefore the fear of the patient when faced with injections or with the fitting of catheters is ascribed mostly to the entering or penetration resistance of the cannula which thus ultimately determines what is known as the entrance quality.
The facts which determine penetration resistance, apart from the condition of the patient's skin, are:
cut-geometry and diameter of the cannula; PA1 quality of finish; PA1 sharpness and intact surface condition of the cannula tip and the cutting edge at the chamfer; PA1 surface condition of the cannula; and, PA1 surface treatment. PA1 the formation of a continuous layer on the cannula surface; and, PA1 good anti-friction properties. PA1 dipping PA1 spraying PA1 printing PA1 rolling PA1 rubbing (with small sponges) PA1 vapor deposition PA1 high-vacuum precipitation. PA1 application by means of small sponges PA1 spraying PA1 rolling PA1 printing PA1 dipping PA1 providing a supply of silicone oil at one side of an elastic foil which is preferably thin and highly elastic; PA1 penetrating the foil on an opposite side to the silicone oil with an uncoated cannula; PA1 wetting the cannula with the silicone oil; PA1 withdrawing the wetted cannula from the foil whilst removing excess oil from the cannula; and PA1 drying the silicone coat.
Damaged or blunt tips cause high resistance in the initial cutting stage, that is to say, in the process of first opening the skin or the vessel. The same applies to blunt cutting edges.
Cutting edges which are too short or blunt produce an incision of inadequate length entailing substantially more severe stretching of the tissues, potentially even tearing at the edges of the incision, but always generating a high circumferential tension and increased friction resistance during the positioning of the needle.
In order to reduce penetration resistance of a cannula it is known to provide the cannula with an anti-friction coating. The absence or inadequate application of surface coating causes greater friction resistance as a result of which the cannula is felt to be blunt by the patient.
It is currently customary to coat the cannula with so called "medical grade" silicone oil which is also listed in the relevant Standards (ISO 7864 (1993), ISO 7885-1 (1996), ISO 10555, ISO 11608, ISO 8537.2).
Most of these Standards contain the instruction that the coating should not be visible when regarded with normal eyesight and must not comprise any droplets or accumulations of silicone oil. ISO 7864 also specifies an upper limit for the permissible amount at 0.25 mg/cm2 of the lubricant relative to the cannula surface. This quantity of silicone must be regarded as the (absolute) upper limit. In view of the fact that during an injection silicone oil could enter into the body, and today free silicone is deemed to constitute a health hazard, it is important to keep the amount of free silicone oil in a cannula or syringe as low as possible.
Undiluted polydimethylsiloxane, having a viscosity of more than 1000 mm2/s and less than 30,000 mm2/s according to the national or European pharmacopoeia, is said to be a suitable and permitted lubricant. Having regard to the health risks of silicone products one endeavors to apply the smallest possible amounts of silicone oil which must however by sufficient to ensure
Another possible way, besides minimizing the amount of silicone applied to the cannula, of keeping the quantity of silicone introduced into the body as low as possible resides in achieving improved adhesion of the silicone to the cannula surface. An example of this is coating with so called reactive silicone which, due to the polar groups, is intended to produce better bonding of the coating layer to the metal surface of the cannula and which by cross-linking produces a cohesive layer.
This type of coating is called "dry siliconization" since it has a rather dry (wax-like) appearance. This type of coating is widely used for large gauge cannulas and for surgical and acupuncture needles. With such dry siliconization only slightly better friction values can be attained at a disadvantage than with wet silicone coating processes. This is due, on the one hand, to the degree of cross-linking, that is to say to the mobility of the coating layer, and, on the other hand, to the thickness of the coating layer. Generally speaking, the pre-condition for lowest friction forces is the presence of a liquid lubricant which in the present case requires the presence of a "free" silicone oil. This is less present in the case of a dry siliconization. If, moreover, in a dry siliconization the layer thickness is too great then a viscous wax-like layer results and friction is slightly increased.
For application of the silicone coating to the cannula in principle all known technical coating methods are available:
However, since cannulas are inexpensive mass produced products it is common practice to use the more expensive siliconization methods only for special high-grade cannula types or packing means with needles or medical devices. The ordinary cannulas or needles are coated by simple methods and means which must be able to be integrated in an automatic assembly machine for mass production; for, as a rule, the cannula itself is initially not siliconized and only coated after assembly of the final product.
Basically there is a distinction applied between coating methods for what are generally called wet siliconizations and those for the aforementioned dry siliconizations.
For the application of wet silicone, the dipping method is widely used as it is simple and can be easily integrated in an automatic assembly process. It has the advantage that the cannula can easily be coated over its entire length simply by dipping or plunging the cannula completely into a silicone bath. Since the liquid level of the silicone container and the descent movement of the cannula can be comparatively easily measured and adjusted, the coating of a cannula over a defined length thereof is reliably adjustable.
Regarding the state of the art in dipping processes we refer, for example, to DE 41 24 909 A1 which describes a dipping method where the silicone solution is continuously fed in such a manner as to prevent thickenings, soiling and sedimentation or demixing. EP 0 494 648 A2 also describes a dipping process of this kind.
However, the silicone layers obtained by a dipping process are often too thick so that, contrary to regulations, they are visible and form droplets. It is particularly harmful that silicone particles may form which may arrive in the body of the patient. The principle "much gives much help" is not true in this case because thickly coated cannulas do not necessarily display the best friction characteristics. If no adequate bonding to the surface is formed the silicone will be displaced so strongly during incision that the friction values are comparatively high despite the thick coating and undesirable silicone bulges or beads tend to be formed.
In order to avoid such effects, it is frequently the practice to use diluted oils in the dipping process. Preferred diluents are halogenated hydrocarbons or alcohols or similar substances or also other organic solvents.
While coating layer quality can be improved by the better and more uniform cross-linking behavior, the solvents are to a considerable extent environmentally harmful as well as presenting risks of fire and explosion accidents. If water is used, on the other hand, a uniform emulsion must be assured which involves high technological expenses; moreover, the water must be removed from the needle surface after coating either by heat treatment or long storage periods.
A further inherent drawback of the dipping method resides in that silicone oil gets into the interior of the cannula and then inevitably also into the patient's body during the injection.
Coating by the methods of rubbing on, rolling on or printing which are equally suitable for automation will produce substantially thinner layers which may however be irregular so that some areas will have a thick coating (or droplet) whilst others are dry because the wetting of the cannula is very difficult to monitor automatically.
While spraying and vapor deposition will produce very even layers these processes take much longer and it is difficult to achieve an accurate setting for the coated surface area of the cannula. Moreover, the appropriate apparatus can be integrated in "clean space areas" only at considerable expense. Their use in final assembly machines and packing installations is therefore rather limited. A spray-coating or an ultrasonic atomizing process of this type was disclosed in EP 0627 474 A1.
The earlier mentioned so called dry silicone coatings are produced with cross-linked silicone oils. The cross-linking of the commonly used silicone oils with reactive fractions predominantly takes place at room temperature or raised temperature and normal air humidity. Alternatively, or additionally, ultraviolet radiation may also be applied. The duration of the cross-linking process will depend on the desired degree of cross-linking and on prevailing environmental conditions. It may be several hours in a thermocabinet or up to several days at room temperature. This means, however, that his kind of siliconization is practically not capable of application in an integrated continuous production on one machine unless the cannula remains exposed to the environment after the process, that is to say, in a container which is permeable by air and moisture, as is the case, for example, with acupuncture needles.
For this reason this type of siliconization is used only with partially, or semi-automatic processes and with pre-assembly machines, e.g. as a final operational step, so that the silicone layer can harden prior to final assembly. Here it is possible to take advantage of the benefits of the spray-coating method, namely the application of an even layer of which the thickness can be controlled by the holding time.
Dipping processes using these kinds of cross-linked silicone oils are, admittedly, also possible but there is a risk that the silicone may already partially cross-link in the dipping tank since there is a relatively large quantity of silicone oil in the tank, that is to say, a multiple of the layer to be applied, as a result of which silicone particles will be deposited on the needle surface.
EP 0 491 547 A1 describes silicone mixes suitable for use in different coating processes such as
and which require no lengthy cross-linking processes. This specification proposes for this purpose the use of so called polar non cross-linking polysiloxanes to which better adhesion to the cannula surface than that of a non cross-linking non-polar silicone coating is ascribed.
Tests have shown that it is not possible with the known methods to realize the relevant requirements according to the Standards, namely to apply a thinnest possible, continuous and even silicone layer to a cannula. In many cases the layer is either continuous and too thick, as occurs frequently with dipping processes, or flawed areas appear. This significantly impairs incision quality of the needle. Moreover, during insertion or placing of a needle cap, bulge or bead formation tends to occur in the silicone layer due to the presence of silicone particles.